The present invention relates to a variable valve timing mechanism for a rotary valve assembly used in an internal combustion engine, and more particularly to a variable valve timing mechanism where both the inlet port and the exhaust port are in the same rotary valve.
Rotary valve arrangements have been proposed by many people. One recent example is that proposed by U.S. Pat. No. 5,526,780 (Wallis). Common to all these valve arrangements is an opening in the rotary valve""s periphery that periodically aligns with a similar shaped window in the combustion chamber. When the opening in the rotary valve""s periphery aligns with the window in the combustion chamber, fluid can pass into (in case of the inlet stroke) and out of (in the case of the exhaust stroke) the combustion chamber. When the opening in the valve""s periphery is not aligned with the window in the combustion chamber the contents of the cylinder are trapped during the compression and combustion stroke.
In most prior art arrangements the rotary valve is driven at a fixed angular velocity ratio to the crankshaft. This is achieved by way of mechanical drive mechanisms such as gear trains, chain drives or belt drives which transmit constant angular velocity ratios.
xe2x80x9cAngular velocity ratioxe2x80x9d is the ratio obtained when the angular velocity of the rotary valve is divided by the angular velocity of the crankshaft. Unless the context requires otherwise any reference made to varying the angular velocity of the rotary valve is made in the context of the angular velocity of the crankshaft remaining constant.
These arrangements all suffer from an inability to vary the engine""s valve timing. The duration of the inlet and exhaust process is fixed by the geometry of the window and the respective openings in the rotary valve. In the event that the rotary valve incorporates both, inlet and exhaust ports in the same rotary valve, the angular phase relationship between the inlet and exhaust process is also fixed by the rotary valve geometry.
This inability to vary the valve timing of rotary valve engines is a significant impediment to its widespread adoption in production car engines. Increasingly, more stringent government regulations in the areas of emissions and fuel economy can only be addressed by internal combustion engines that have the ability to vary the engine""s valve, timing.
Valve timing is generally expressed as the location of the inlet open, inlet close, exhaust open and exhaust close points relative to the crankshaft position. The crankshaft position is generally specified as an angle relative to a reference location. This is generally chosen to be the location where the piston is at the top of its stroke (i.e. top dead centrexe2x80x94tdc). If the exhaust closes 15xc2x0 after tdc the exhaust port will cease communication with the cylinder when the crankshaft has rotated 15xc2x0 from the position where the piston was at top dead centre.
Alternatively valve timing can be thought of as a combination of durationsxe2x80x94inlet duration, exhaust duration, close duration and overlap duration, together with an initial position and phase. The initial position determines the relationship between the crankshaft position and the rotary valve position at some point.
xe2x80x9cOverlapxe2x80x9d is that portion of the engine cycle where both inlet and exhaust ports are both simultaneously open to the combustion chamber.
xe2x80x9cDurationxe2x80x9d is the angle the crankshaft rotates through between any two events.
xe2x80x9cInlet durationxe2x80x9d is the angle the crankshaft rotates through when the inlet port is in communication with the combustion chamber i.e. between inlet open and inlet close. Similarly xe2x80x9cexhaust durationxe2x80x9d is the angle the crankshaft rotates through when the exhaust port is in communication with the combustion chamber i.e. between exhaust open and exhaust close. xe2x80x9cClose durationxe2x80x9d is the angle the crankshaft rotates through when neither the inlet nor the exhaust port are open to the combustion chamber i.e. between inlet close and exhaust open. This occurs during the compression and power strokes on a four-stroke, engine. xe2x80x9cOverlap durationxe2x80x9d is the angle the crankshaft rotates through when both the inlet and exhaust ports are simultaneously open to the combustion chamber i.e. between inlet open and exhaust close.
In all internal combustion engines synchronization of the valve events to their correct position in the engine cycle is essential. Phase is used to describe this synchronization. If the phase is constant from cycle to cycle the valve events will occur in exactly the same position in the cycle from one cycle to the next.
The position in the cycle is defined by the crankshaft position. The position of the rotary valve is described by the angle the valve has rotated from a reference location usually chosen as one of the easily observable valve eventsxe2x80x94i.e. inlet valve open (ivo), inlet valve close (ivc), exhaust open (evo) or exhaust valve close (evc). For ease of reference in this specification we have chosen the reference location to be ivo. xe2x80x9cRotary valve positionxe2x80x9d is defined as the angle the valve has rotated from the ivo point.
For conventional rotary valve internal combustion engines using drive mechanisms that deliver constant angular velocity ratio the position of the rotary valve relative to the cycle position can be represented by a graph of the type shown in FIG. 10A. Line 56 defines the position of the rotary valve for all crankshaft positions. So long as the relationship defined by this line occurs on successive cycles, phase has remained constant (and equal to zero i.e. "sgr"=0). In the event the relationship between rotary valve position and crankshaft position is at some other time represented by line 57 a phase change is said to have occurred and its magnitude is "sgr". In the event line 56 is chosen as the reference, the phase is a "sgr"xc2x0.
xe2x80x9cPhasexe2x80x9d in this context is the distance in crankshaft degrees (xc2x0 crankshaft) that the line 57 defining constant phase has shifted relative to reference line 56 defining (nominally) zero phase.
In the event an arrangement was used where the drive mechanism delivers a varying angular velocity ratio during the cycle, another relationship represented by lines 58 in FIG. 10B may occur. So long as the mechanism maintains this relationship from cycle to cycle there is no phase change. In the event the rotary valve at some other time follows the relationship shown by lines 59 a phase shift of "sgr"xc2x0 has occurred and, in the event lines 58 are the reference lines (nominally) defining zero phase, the phase will be a "sgr"xc2x0.
In the event the drive mechanism can vary the angular velocity ratio within the cycle and also vary the shape of the angular velocity ratio curve plotted against crankshaft position from cycle to cycle we have a situation shown in FIG. 10C. Lines 60 represents the relationship between valve position and cycle position in one cycle and lines 61 represent the same relationship in the next cycle. Clearly there is a change in synchronization at all points within the cycle. Instantaneous phase changes are occurring at all points within the cycle (at rotary valve position 120xc2x0 a phase change of "sgr"xc2x0 has occurred). However the relationship between rotary valve position and cycle position is unaltered at the start and end points of the cycle i.e. there is no change in synchronization or phase at these points.
FIG. 10D shows an alternative outcome from this mechanism. Lines 62 represents the relationship during the first cycle and lines 63 represents the relationship during the next cycle. In this case the end point of the cycle represented by lines 63 is different from the end point of the previous cycle. The phase has been changing within the cycle and at the end of the cycle i.e. there is a change of synchronization both during the cycle and at the end of the cycle.
To distinguish between an arrangement that maintains phase at the start and end of each successive cycle and one that doesn""t and also to distinguish between an arrangement which maintains phase at the start and end of each cycle but varies the phase within the cycle and one that doesn""t vary the phase within the cycle the following definitions apply.
xe2x80x9cPhasexe2x80x9d is defined, as the change in crankshaft position for any specified rotary valve position relative to the crankshaft position on the reference cycle or the reference; curve defining this cycle.
xe2x80x9cCyclic phasexe2x80x9d is the phase determined by a consideration of the phase at the start and end points only of each cycle and each successive cycle.
In FIGS. 10A and 10B there is no change in phase whilst ever successive cycles sit over the op of lines 56 and 58 respectively. In FIG. 10C there is no cyclic phase change between cycles represented by lines 60 and 61 despite the fact there are changes in phase within the cycle. In FIG. 10D there is a change in cyclic phase of xcex2xc2x0 between the cycles represented by lines 62 and 63 and changes in phase within the cycle also.
As the definition of start and end points of a cycle is purely arbitrary cyclic phase is defined as remaining constant over that cycle whenever a pair of start and end points separated by one cycle can be found that has the same phase. Cyclic phase is defined as being constant over consecutive cycles when the phase at the end of the next cycle coincides with the phase at the end of the previous cycle. The phase of the rotary valve between the start and end of the cycle may vary between cycles but, whilst ever the phase at the end of consecutive cycles is unaltered, the cyclic phase remains the same. In the case of rotary valves where the angular velocity ratio varies during the cycle, xe2x80x9ccyclic phase changexe2x80x9d is defined as having occurred whenever two points (start and end points) separated by a cycle can""t be found having the same phase and/or the end points of consecutive cycles do not have the same phase.
Various means have been proposed to introduce variation into the valve timing of rotary valves. U.S. Pat. No. 5,205,251 (Conklin) describes a means of varying the valve timing of a rotary valve engine fitted with two rotary valves per cylinder. One rotary valve contains an inlet port and the other rotary valve contains an exhaust port. The rotary valves are housed inside sleeves and are able to rotate within these sleeves. The sleeves are rotatably disposed within the cylinder head. Timing variation of the inlet or exhaust events is achieved by a combination of rotation of the sleeves and variation of the rotary valves"" angular velocity during the cycle.
In this arrangement the variation in the rotary valves"" angular velocity during the cycle varies either the inlet and/or the exhaust duration. As this mechanism can vary the magnitude of the duration from one cycle to the next it is also able to introduce phase changes within the cycle. These phase changes must however be symmetric about some selected point in the cycle. This restricts the ability of the mechanism to effect usefully significant phase changes.
As this mechanism is unable to introduce changes in cyclic phase, an additional sleeve mechanism is required to vary the phase in a useful manner. The rotation of the sleeve varies the location of the inlet and/or the exhaust events relative to the crankshaft or the phase of these events relative to the crankshaft. The combination of variations in the angular velocity of the rotary valves and rotation of the sleeves allows independent movement of the inlet open, inlet close, exhaust open, and exhaust close points.
The provision of a sleeve and an additional mechanism to vary its location is additional complication and also introduces additional gas sealing difficulties. U.S. Pat. No. 5,205,251 remains silent on how gas sealing is achieved. However it is clear that gas sealing will be required between the combustion chamber and the sleeve and between the sleeve and the rotary valve. There is no known practical solution for this arrangement and the requirement to seal in two places merely increases the complexity.
Any arrangement that varies the timing by use of a sleeve requires a window in the cylinder head that is wider than the opening in the valve. This is well illustrated in FIGS. 2 and 5 in U.S. Pat. No. 5,205,251. As the breathing capacity of the rotary valve is determined in part by the width of the opening in the rotary valve, there is no practical requirement for the window in the head to be wider than the rotary valve opening apart from that introduced by the use of the sleeve. The wider window in the cylinder head is a problem from several aspects. Firstly the gas loads imposed on the rotary valve during combustion are directly proportional to the cylinder head window width and are therefore unnecessarily high in the case of applications using sleeves to vary timing. Secondly the volume occupied by these windows is unnecessarily high and makes design of combustion chambers having the required compression ratios difficult.
As stated earlier, the mechanism designed to vary the angular velocity ratio during the engine cycle as-disclosed in U.S. Pat. No. 5,205,251 suffers from the fact that it is only capable of producing symmetric angular velocity ratio profiles. It is the inability of this mechanism to produce an asymmetric angular velocity ratio profile that introduces the requirement for the additional sleeve mechanism.
xe2x80x9cAngular velocity ratio profilexe2x80x9d means the locus of the rotary valve""s angular velocity ratio points plotted against crankshaft angle.
A xe2x80x9csymmetric angular velocity ratio profilexe2x80x9d is one where a point on the crankshaft angle axis of the angular velocity ratio profile can be found where the angular velocity ratio profile extending over half a cycle either side of this point is symmetric.
An xe2x80x9casymmetric angular velocity ratio profilexe2x80x9d is one where no point on the crankshaft angle axis of the angular velocity ratio profile can be found where the angular velocity ratio profile extending over half a cycle either side of this point is symmetric.
An example of a symmetric angular velocity ratio profile is shown in cycle C6 in FIG. 8A. The angular velocity ratio profile of cycle C6 is mirror symmetric about the axis through point 29.
An example of an asymmetric angular velocity profile is shown in cycles C1, C2, and C3 in FIG. 9A. No point can be found within the 0xc2x0 to 2160xc2x0 crankshaft angle range where a symmetric angular velocity profile exists over a cycle centred on the point. Another example is shown in FIG. 11.
U.S. Pat. No. 5,711,265 (Duve) describes, a drive mechanism where the angular velocity of the valve is varied during the engine cycle. In this mechanism the valve is indexed to various positions where it remains stationary for a predetermined period before being indexed to its next position. In this arrangement the phase and duration of these events are predetermined by the design of the cam mechanism and are consequently unable to be varied.
The present invention improves the variable valve timing mechanism of an internal combustion engine.
It varies from previous variable valve timing arrangements in that it applies to rotary valve arrangements where both the inlet and exhaust ports are housed in a single rotary valve. This arrangement has the major advantage that a single rotary valve can perform the function of two rotary valves with half the number of components.
Unlike the invention disclosed in U.S. Pat. No. 5,205,251, the change in valve timing is achieved by a single mechanism i.e. it doesn""t require a separate sleeve and drive mechanism for this sleeve, to achieve phase change.
Also unlike the invention disclosed in U.S. Pat. No. 5,205,251, the present invention requires sealing only between the combustion chamber and the valve and practical solutions are well known in the art (see U.S. Pat. No. 5,526,780). The variable valve timing, mechanism can be applied to any rotary valve arrangement irrespective of its gas sealing details.
A single rotary valve incorporating both inlet and exhaust ports in the, same valve is a substantial improvement over arrangements requiring separate valves for the inlet and exhaust ports. The following considerations make this clear.
Two important features relevant to all valve mechanisms for internal combustion engines are the rate at which the valve opens and closes and the maximum breathing capacity of the valve system. In the, case of rotary valves the length of the window in the cylinder head and the valve diameter-determine the rate at which the valve opens and closes. The length of the window is geometrically constrained by the requirement to have it located within, the bore of the cylinder and can be made a similar length whether there are one or two valves per cylinder. The maximum breathing capacity is determined by the valve diameter. Thus, for the same maximum breathing capacity, the valve diameter for the rotary valve with a single inlet port must be the same as the valve diameter for a valve with both inlet and exhaust ports in the same valve. Consequently a single valve incorporating both the inlet and exhaust ports in the same valve will have the same maximum breathing capacity and open and close rates (i.e. the same breathing capacity) as will two rotary valves incorporating inlet and exhaust ports in separate valves but with half the number of components.
In addition, a twin rotary valve incorporating inlet and exhaust ports in separate valves will have twice the number of bearings and seals as required with a single valve incorporating both inlet and exhaust ports in the same valve. Consequently friction losses in the two-valve arrangement are potentially double those in the single valve arrangement with both inlet and exhaust ports in the same valve.
In the event that other considerations require the use of two valves per cylinder, a rotary valve incorporating both inlet and exhaust ports in the same valve will have twice the opening and closing rate for the same window length as will two rotary valves of the same diameter incorporating only a single port in each valve. In this case the arrangement with both inlet and exhaust ports in the same valve will have twice the maximum breathing capacity. Consequently two valves incorporating both inlet and exhaust ports in the same valve will have twice the breathing capacity of two valves of the same diameter incorporating the inlet and exhaust valves in separate rotary valves.
Whilst attempts have been made to address the issue of variable valve-timing in rotary valve arrangements where the inlet port and exhaust port are accommodated in separate rotary valves, no attempts have been made to address the inherently more difficult arrangement where both the inlet and exhaust ports are accommodated in the same rotary valve.
This added difficulty arises because, in this latter arrangement, the angular phase relationship between the exhaust events and the inlet events are fixed by the geometry of the rotary valve. A simple phase change between a rotary valve incorporating both inlet and exhaust ports and the crankshaft cannot therefore effect a change in the location of the inlet and exhaust relative to each other. By way of comparison the use of separate rotary valves for the inlet port and the exhaust port means a simple phase change between one or both of the rotary valves and the crankshaft will change the phasing between the inlet and exhaust and will alter the overlap.
Varying the angular velocity of the valve within the cycle can effect significant changes in the duration of those events that have long duration relative to the desired change in duration. For example both inlet and the exhaust events have relatively long durationxe2x80x94typically 230 crankshaft degrees and a significant change in timing is in the order of 40 degrees or 17% of the duration. Consequently significant changes in duration can be achieved by moderate changes in rotary valve angular velocity.
This is not the case with overlap duration. Overlap duration is typically 40xc2x0 and a significant change in this duration is in the order of 40xc2x0. In a rotary valve incorporating both inlet and exhaust ports in the same rotary valve and a cylinder head with fixed window geometry, there is limited ability to effect changes in the overlap duration irrespective of what strategy is adopted.
For example in the extreme arrangement where the rotary valve has zero overlap duration, the bridge on the outer diameter of the valve spanning between the inlet port and the exhaust port has an identical width to that of the window in the cylinder head. As the geometry of both the head and the rotary valve is fixed, there is clearly no way of introducing overlap.
In more conventional arrangements with non-zero overlap duration, small changes could be effected by varying the angular velocity of the rotary valve during the overlap period. If the rotary valve""s angular velocity is slower than normal during the overlap the duration will effectively be increased. If the rotary valve""s angular velocity is greater than normal during overlap the overlap duration will be decreased. If for example the average rotary valve angular velocity during the overlap was double the average rotary valve angular velocity the overlap duration would be halved. This method is of limited use as large changes in angular velocity effect relatively small changes in overlap. To vary the overlap duration by varying the rotary valve angular velocity during overlap will have very little effect on the overlap duration and will have diminishing effect as the nominal overlap duration decreases.
No practical mechanism of this type could effect a change from say 40xc2x0 overlap duration to zero overlap durationxe2x80x94a typical requirement. This is an inherent limitation in any rotary valve incorporating both inlet and exhaust ports in the same valve ie. there is limited ability to alter the overlap duration. The present invention uses another strategy to overcome these limitations.
In addition, the presence of both the inlet and exhaust ports in the same rotary valve introduces other constraints not present when the inlet and exhaust ports are in separate rotary valves. As both the inlet port and the exhaust port have a fixed geometric relation to each other it is not possible to introduce changes in duration of one port without considering the impact of such a change on the other port. For example, if the inlet duration was decreased by increasing the rotary valve""s angular velocity, a compensating reduction in the rotary valve""s angular velocity will have to be made elsewhere in the engine cycle. If this reduction in angular velocity occurs during the exhaust cycle it will increase the duration of the exhaust. If it is desired to leave the duration of the exhaust unaltered (ie the average rotary valve angular velocity during the exhaust stroke is equal to the average angular velocity of the rotary valve over the engine cycle) then the rotary valve""s angular velocity must be reduced during the compression and combustion portion of the cycle. The angular velocity ratio profile for this strategy is shown in FIG. 11. It is clearly an asymmetric angular velocity ratio profile.
In order to cater for changes in valve timing in a rotary valve with both inlet and exhaust ports in the same valve, by altering the rotary valves"" angular velocity, it is clearly necessary to have a valve drive mechanism capable of producing an asymmetric angular velocity ratio profile. It is a fundamental requirement for a variable valve timing mechanism to produce asymmetric angular velocity profiles if this mechanism is to produce usefully significant phase changes as well as duration changes.
According to a first aspect the present invention consists in a variable valve timing mechanism for an internal combustion engine, said engine comprising a crankshaft, a cylinder head, a combustion chamber, and at least one rotary valve, said rotary valve having at least two ports terminating as openings in its periphery, said cylinder head having a bore in which said rotary valve rotates, a window in said bore communicating with said combustion chamber, said openings successively aligning with said window by virtue of said rotation, and a drive mechanism driving said rotary valve, characterised in that said at least two ports comprise an inlet port and an exhaust port, and said drive mechanism varies the angular velocity of said rotary valve at least within a portion of at least one engine cycle whilst maintaining an average angular velocity over said at least one engine cycle that has a fixed relation to the average angular velocity of said crankshaft over said at least one engine cycle, and wherein said drive mechanism produces either a symmetric or assymetric angular velocity ratio profile for said rotary valve with respect to said crankshaft.
Preferably said drive mechanism intermittently varies the angular velocity of said rotary valve over one or more other engine cycles such that the average angular velocity over said one or more other engine cycles varies from said fixed relation.
Preferably said drive mechanism comprises an electric motor.
In a first embodiment said electric motor is directly coupled to said rotary valve.
In a second embodiment said electric motor drives one or more intermediate drive members operably engaged with said rotary valve. Preferably said one or more intermediate drive members comprises any one of a gear, gear train, chain drive assembly or a belt drive assembly.
In a third embodiment said drive mechanism comprises a primary drive means for transmitting motion between said crankshaft and said rotary valve, said primary drive means having at least one epicyclic gear set, and a secondary drive means driving a sun gear of said epicyclic gear set.
In a fourth embodiment said drive mechanism comprises a primary drive means for transmitting motion between said crankshaft and said rotary valve, said primary drive means having at least one epicyclic gear set, and secondary drive means driving one or more planet gears of said epicyclic gear set.
Preferably said secondary drive means is an electric motor.
Preferably said drive mechanism is operably connected to an electronic control unit which controls the angular velocity of said drive mechanism and hence the angular velocity of said rotary valve.
Preferably said secondary drive means is operably connected to an electronic control unit which controls the angular velocity of said secondary drive means and hence the angular velocity of said rotary valve.
According to a second aspect the present invention consists in a variable valve timing mechanism for an internal combustion engine, said engine comprising a crankshaft, a cylinder head, a combustion chamber, and at least one rotary valve, said rotary valve having at least two ports terminating as openings in its periphery, said cylinder head having a bore in which said rotary valve rotates, a window in said bore communicating with said combustion chamber, said openings successively aligning with said window by virtue of said rotation, and a drive mechanism driving said rotary valve, characterised in that said at least two ports comprise an inlet port and an exhaust port, and said drive mechanism is controlled by a control means to vary the angular velocity of said rotary valve, said control means controlling said drive mechanism in response to sensed engine parameters to produce variations of the angular velocity of said rotary valve at least within a portion of at least one engine cycle whilst maintaining an average angular velocity over said at least one engine cycle that has a fixed relation to the average angular velocity of said crankshaft over said at least one engine cycle, and wherein said drive mechanism produces either a symmetric or assymetric angular velocity ratio profile for said rotary valve with respect to said crankshaft.
Preferably said control means is an electronic control unit.